1. Field of the Invention
The present invention relates to an optical detection apparatus and an optical detection method for measuring with a high resolution an image in the wavelength range of from the infrared region to the gamma-ray region.
2. Related Background Art
There are known optical detection apparatus for optically detecting an image with a high resolution, for example, X-ray telescopes, and X-ray microscopes for obtaining an X-ray image under irradiation of X-rays on a subject. For example, an X-ray microscope using soft X-rays with wavelength ranging from 2 nm to 5 nm can achieve a higher resolution than one using visible light (with wavelength ranging from about 400 nm to 800 nm) and permits in observation of a live organism sample in water, which is thus useful in the fields of medicine and biology. Further, an X-ray microscope is constructed in general of an X-ray generator, a condenser optical system for collecting X-rays emitted from the X-ray generator to illuminate a sample to be observed, a focusing optical system for focusing an X-ray image transmitted by the sample, image pickup means positioned at the focus position of the focusing optical system to pick up the X-ray image, a vacuum receptacle enclosing an optical path between the X-ray generator and the image pickup means in vacuum, and an evacuation system for evacuating the vacuum receptacle.
Such X-ray microscopes were arranged to realize two-dimensional image pickup by employing, as the image pickup means located at the focus position, a photosensitive medium such as a silver salt film or a photoresist, an image pickup device in which a micro-channel plate (MCP) electron-intensifies the X-ray image and thereafter a fluorescent plate converts the X-ray image into a visible image, or a CCD solid-state image sensing device.
The image pickup device employing the CCD solid-state image sensing device or the micro-channel plate, however, had such a problem that a resolution of pixels for sensing the X-ray image was extremely lower than the resolution of X-ray image obtained in the wavelength range of X-rays, and that a finally observed image could not be formed at a high resolution despite that the X-rays were employed for obtaining a high resolution. There is a limit in miniaturizing the pixels in image pickup device. Specifically, the minimum size of a pixel is in the range of about 10 .mu.m to ten and a few .mu.m. Further remarkable miniaturization seems difficult at present. It is also conceivable that the resolving power of an image pickup device can be relatively increased by employing a high-magnification focusing optical system. It could instead raise another problem of increase in size of optical systems (e.g., the focusing optical system), which results in increasing the size of the entire optical detection apparatus, in addition to the difficulty in producing X-ray optical elements.
In case that a photosensitive medium such as a silver salt film or a photoresist is used as the image pickup medium, a high resolution can be obtained to some extent. However, it had a problem of worse operability, for example a relatively longer time necessary for development, and troublesome works necessary for taking a photosensitive medium out of the vacuum receptacle and setting another one at every observation.
Developed as means for solving such technical problems in the conventional apparatus was an X-ray image detecting apparatus utilizing the so-called tunneling, which detects photoelectrons produced in a photoelectric surface upon incidence of X-ray image by means of a conductive probe arranged apart from the photoelectric surface. It is described for example in Japanese Laid-open Patent Applications No. 2-123700, No. 4-152289, No. 5-52780. In a typical example as described in Japanese Laid-open Patent Application No. 4-152289, a probe two-dimensionally scans a photoelectric surface in a non-contact state and a current flowing into the probe due to the tunneling is detected to obtain a distribution of photoelectrons which increase in proportion to an X-ray irradiation intensity in each portion in X-ray image, whereby the X-ray image is two-dimensionally detected theoretically at an extremely high resolution.
Such means also has a big problem to be solved. Namely, a very serious point is that in order to achieve a high resolution, two-dimensional scan must be carried out while keeping the tip of probe opposed to the photoelectric surface (surface to be detected) at a constant and very small clearance therebetween (for example at a clearance of ten angstroms). Such means can attain a theoretically very high resolution, but the non-contact scan is very difficult even with high-precision servo system. Actually, the photoelectric surface is uneven, or the clearance varies between the probe and the photoelectric surface during scan, which causes a change in current, an amount of which is information to be detected, according to the clearance variation (which means that the detection sensitivity varies depending upon the scanned portion). Thus, the detected current includes errors, which lowers the detection accuracy. Accordingly, it was practically impossible to make the clearance between the photoelectric surface and the probe very small and, therefore, high resolution optical detection has not been achieved.